Of late, the external appearance and finish of plastic molding used in automobiles and home electric appliances have come to be highly valued in order to enhance luxury as well as to increase an added-value of commercial merchandise. Particularly, resin molding that is molded by injection molding or the like can have a variety of facades depending on die surface decorations. Conventionally, of the die surface decorations in molding, there has been known an etching method by which to corrode the die through the functions of chemical agents, making it possible to form on the die surface a variety of patterns such as satin finished surface, leathery surface, grain woody texture, linear embossing, geometric patterns or the like. In addition, there is known a sand-blast (honing) processing that forms a fine pattern on the surface of an object by physically blasting and glass beads.
On the other hand, there is known a method of exposing a metal part of the portion necessary for etching or similar by spraying a photoresist in liquid form across the whole of a die and sticking a photoengraved film-like photomask to the die to be exposed and developed.
Hereinafter, a method using a conventional film-like photomask will be explained with reference to FIGS. 3.
First, as shown in FIG. 3A, photomasks 11, 12, and 13 are made by cutting off a photoengraved film-like photomask according to a plan view spread out based on the shape of a die. Then, as shown in FIG. 3B, the cut-off photomasks 11, 12, and 13 are stuck to a die 20 coated with a photoresist, and seams of the photomasks are joined together by tapes or the like to be exposed and developed. Next, the developed condition of the photoresist is examined, and after discontinuity in the joined parts of the photomasks is corrected, etching is performed to form a pattern on the die 20.
Meanwhile, there is known another method of directly exposing a photoresist coated on a die by using laser beams.
In Japanese published patent application No. 51-63866 published by the Japanese Patent Office, a method is disclosed in which concavity and convexity are formed by an etching process on the core side surface of a die used for injection molding.
However, the decorations by the etching method and sand-blast process (honing) have been rarely applied to the whole of the die, so that it is necessary to cover and hide the parts to which it is unnecessary to apply thereof by adhesive tapes for masking or the like. There are many cases where it depends on manual work such as when the die has a complicated three-dimensional configuration and when it has parts in need of fine work such that it caused a lot of trouble and there was a limit in its process.
On the other hand, the masking method that uses photoresist has a high feasibility of molding a pattern with high accuracy when it is compared with the above-mentioned masking method. However, when it comes to the case where a pattern needs to be incised on the surface of a die, the photomask must be cut apart and then joined with each other on a three-dimensional die, which leads to an occurrence somewhere of a seam between the photoresist, making perfect transcription impossible. As a result, as shown in FIG. 3C, there remains a seam 20a in the pattern on the surface of the die, which is a problem in terms of exterior appearance. Further, there is no other way to join the photoresist than to be dependent on manual work, which requires a lot of trouble.